Starting point is a Synology DS923+ which is configured with a single 22 TByte storage pool, 10Ge interface but only 4 GBytes RAM.
When installing this system I included two spare 4 TByte, Samsung, Gen4 NVMe drives. The NAS formatted the 4 TByte drives as a 3.6 TBytes RAID1 cache and reported 3.8 GBytes was required to store the btrfs meta data.
If you use the Synology RAID calculator it will report that two 4 TByte drives in RAID1 will be formatted as 3.6 TBytes. So if you know your required cache capacity then you can play with the calculator to determine what capacity drives are required. Catch is determining the minimum cache size for btrfs meta storage.
Synology provides an online RAID calculator for checking possible drive configurations. Is there a calculator for cache configurations or a method to determine the minimum cache size for the btrfs meta data?
Joke is with the move to Gen5 drives the 4 TByte Gen4 drives were left overs and only installed because they had no other uses.